Optical waveguides have a wide range of application; they are used, for example, in light-transmission arrangements or light-transmission systems such as optical communication systems, and they are predominantly made of a type of glass which has a high silicon-dioxide content (which, if required, contains a dopant for adjusting the refractive index of the glass).
Types of glass which can suitably be used for the manufacture of optical waveguides can also advantageously be used for the manufacture of lamp envelopes for halogen lamps or gas-discharge lamps, because these types of glass, like the ones used for optical waveguides, must be substantially anhydrous and exhibit a high silicon-dioxide content.
GB No. 10 10 702 describes a method in which powdered, pure or substantially pure SiO.sub.2 to which a liquid binder is added in a quantity from 1 to 50% by weight of the SiO.sub.2 content, and to which optionally a further lubricant which facilitates the extrusion process may be added in a quantity from 0.1 to 10% by weight of the SiO.sub.2 content, are worked into an extrusion mass which is subsequently deformed in an extrusion process. For example, organic fluids such as polyvinyl alcohol or methyl cellulose may be used as liquid binders.
This method corresponds to methods used in the art of ceramics, in which ground, powdered starting materials having grain diameters &gt;1 .mu.m, with water as a dispersing liquid, with binders and optionally with lubricants are worked into very viscous extrudable masses.
In processing microdispersed starting powders having grain diameters &lt;1 .mu.m (which cannot be formed in conventional grinding porcesses), as they are used for the manufacture of quartz glass bodies, in particular for preforms of optical waveguides, problems will arise with the mixing or kneading, particularly, when the starting materials have a high solid content, because in this case a considerably larger quantity of particles must be uniformly distributed, and a considerably larger surface must be uniformly covered with the additives (binders and optionally lubricants). For example, the typical mixing and kneading times for a starting material of microdispersed SiO.sub.2 particles (10-100 nm diameter) having 60% by weight of SiO.sub.2 and 36% by weight of water (the rest are additives) are approximately from one to three hours.
Besides the fact that homogenization of very viscous suspensions (extrusion masses) of the latter type is very time-consuming, there are other problems: In the case of unsatisfactorily stabilized suspensions a segregation tendency will readily develop which leads to extrusion problems. This can be accredited to the substantially enlarged surface of the solid particles in such suspensions, to the growing number of solid-liquid interfaces as the particles become smaller, and to the conditions of linkage (the van der Waal's forces being of growing importance and colloid-chemical aspects manifesting themselves). Minor, local variations in the green body density and, hence, in the pore density readily lead to the formation of cracks during drying and to deformations during sintering. The latter occurs in particular when molded glass bodies are sintered via viscous flow. For example when the extruded bodies are fed in another direction, as is customary for the continuous production of small-diameter bodies in ceramics technology, this has a tendency to affect the homogeneity of the pore structure, ultimately manifesting itself in deformations of the moulded body after the sintering process.